1. Field of the Invention
The present invention relates to Automatic Meter Reading (AMR).
2. The Prior Art
AMR systems fall into two types. There are systems that are designed to provide the readings on demand and fixed infrastructure systems. In xe2x80x9con demand systemsxe2x80x9d a meter is interrogated with a hand-held computer, a passing vehicle or from fixed network system. In each case the responses are obtained within a few seconds of the request. Clearly, this is essential with hand-held and mobile systems that need to complete the transaction while still in the vicinity of the meter.
In AMR fixed infrastructure systems, the data is sent to and received from neighborhood xe2x80x9cnodesxe2x80x9d. The path between the nodes and the meters is available continuously. Therefore, quick communication is less important. On demand readings can be provided with fixed infrastructure systems. However, this is usually a matter of operational convenience, such as the ability to read a consumer""s meter while speaking to them on the telephone. An historical database showing the consumer""s consumption over the previous few days could provide the same customer service. In general, for billing purposes, a reading or set of readings received once per day for each meter is all that is essential for accurate billing.
Technical Problems
One Way/Two Way Operation
Mobile AMR systems have to send some sort of wake up or interrogate signal to the meter to request the reading. This means that such systems always need a two-way radio system. This requires a transmitter and receiver at both the meter and the node.
The perceived advantage of two-way systems is the ability to send data to the meter. In practice, this data is usually simply clock signals to update the internal clock in the meter to ensure that readings are taken at the right time. Other operational requirements like altering the tariffs can be achieved by post processing operations on the received data, provided that sufficient accurately timed readings are available. Two-way communications can also provide information to the consumer such as the fact that the billing tariff has changed.
The present invention is primarily concerned with the use of fixed network AMR systems.
Range
A major cost and difficulty in installing fixed network AMR systems is finding suitable sites which can have the necessary antennae, phone lines and main power available for the nodes. In most parts of the world there are constraints on how much radio power can be used for the communication between the node and the meters. The use of high power is also expensive and complicated. In practice, with conventional narrow band radio techniques the range between the meter unit and the node is approximately a 1,000 m. This means that in a medium-sized town, say 5 km by 5 km, a minimum of 25 nodes would be required. Clearly it is desirable to reduce the number of nodes required. Doubling the range of each node would give a fourfold reduction in the number required. In this example the ideal situation would be to achieve slightly greater than 5 km range. In that case only one node would be needed for the whole town.
The range between the meter unit and the node cannot be increased by increasing the transmitted radiated power because that is limited by the licensing authorities. Therefore, the only method of increasing the range is by improving the receiver sensitivity. In free space the range of a radio system obeys the inverse square law. Therefore, to get five times the range an improvement in sensitivity of 25 times is needed. In an urban environment with the antenna relatively close to the ground, the relationship between range and sensitivity is a fourth power law.
Therefore, for five times range increase a 54=625 times improvement in sensitivity is needed. In radio terms this is a 28 dB increase in receiver sensitivity.
Minor improvements in the receiver design, such as optimizing the noise figure, might give a few dB of improvement. This will not achieve the required result. Most modulation schemes in common use, such as AM, FM and PM need recovered signal to noise ratios in the range of 10 to 15 dB. It is unlikely that these could be improved to give the necessary sensitivity.
Another possibility is to reduce the receiver/noise bandwidth in order to give the required sensitivity improvement.
A conventional narrow band FSK radio system with a 4800 baud rate would probably have a signal/noise bandwidth of about 25 kHz. Improving this to 32 Hz would be necessary to provide the necessary improvement in sensitivity to achieve the target range of 5 km.
Reducing the bandwidth by this ratio also means the data rate needs to be slowed down by the same amount giving a data rate of 7.68 bits per second. A 128 bit message will now take nearly 17 seconds to send. For a fixed network AMR system this is not a serious problem. A full set of 48xc2xd hourly readings each of 16 bits would still only take two minutes to transmit.
Frequency Stability
The problem with reducing the bandwidth is that the frequency stability needs to be increased so that the weak signal stays within the narrow channel. In the example above, a 32 Hz bandwidth requires a frequency stability of the order of 5 Hz. At a typical operating frequency of 184 MHz this means that the combined stability of the transmitter and receiver must be better than 0.027 ppm. This cannot be achieved with any existing oven controlled or temperature compensated crystal oscillator.
Techniques such as automatic frequency control (AFC) will not help because they require a sufficiently close xe2x80x9con-tunexe2x80x9d so that there is enough energy from the wanted signal to drive the AFC detectors.
The present invention overcomes the technical problems of increasing meter to node range by eliminating the need to synchronize transmitter and receiver frequencies by exchanging data between them. Instead, the present invention uses a common frequency reference source for both transmitter and receiver. The common frequency reference source must be available at all meters and at the receiving node. In the UK there are a number of transmitters that could be used for this purpose. For example the MSF Rugby transmission on 60 kHz, the BBC 198 kHz transmitter at Droitwich and any local FM station which carries radio data service (RDS) or a local wide area paging transmitter.
Similar common reference source transmitters are also available elsewhere in the world. For example the LF/MF services from WWV in Boulder Colorado would be ideal for AMR applications in the USA.
The advantage of MSF, Droitwich, WWV or RDS is that these common reference sources already send time data information as part of their normal operation. The VHF FM RDS transmitters have the disadvantage that they are at VHF making the receiver and the decoder more expensive. The LF transmitters are simpler to receive, particularly for underground pit mounted meters. A low-cost microcontroller can be used for the decoding. The BBC. Droitwich transmitter is already used for providing control and timing for radio time switches used in the UK domestic electricity industry.
Advantages of the Invention
The provision of a fixed network AMR system with a significant range makes this a viable alternative to the on demand systems that have previously been thought essential. Previous techniques for increasing sensitivity in order to increase range have focused on solutions that cannot deliver the necessary improvement. By recognizing that a publicly available stable frequency transmitter can be used as a common reference source, the present invention offers an economic solution which still allows low cost transmitters to be used. The reduction in data rate is not a significant problem as it is still possible to provide an enhanced consumer service relative to that provided by personal meter reading. Similarly, the absence of two way communications can also be overcome as time data is available from the common reference source.